Add option to validate the body as a json

[rodrigowirth]

This is a proposal for the issue #122

[dougwilson]

Hi @rodrigowirth it is not clear what this PR is even doing, even reading the docs and source. Can you give a much, much more detailed explanation of exactly what this PR does? It looks like, to me, all it does is prepend a word to the error message with the use of a switch?

[dougwilson]

Oh, nevermind, I see the res change. Unfortunately, no, this PR is not going to be accepted. We ujse the standard error handing from Express and middleware; the module is never going to have functionality to write to the res object.